FIGS. 1A and 1B are block diagrams illustrating a transmitter and a receiver of a conventional Orthogonal Frequency Division Multiple Access (OFDMA) system, respectively. The OFDMA system uses an orthogonal frequency division multiplexing (OFDM) scheme. The OFDM scheme divides a high rate data sequence into a plurality of low rate data sequences, and simultaneously transmits the plurality of low rate data sequences using a plurality of orthogonal subcarriers. The OFDMA implements a multiple access by providing each user with some parts of available subcarriers. In an uplink, a transmitter may be used as a part of a user equipment (UE), and a receiver may be used as a part of a base station. In a downlink, a transmitter may be used as a part of a base station, and a receiver may be used as a part of a user equipment (UE).
As shown in FIG. 1A, the OFDMA transmitter 100 includes a constellation mapping module 102, a Serial/Parallel (S/P) converter 104, a symbol to subcarrier mapping module 106, an Nc-point Inverse Fast Fourier Transform (IFFT) module 108, a Cyclic Prefix (CP) module 110, and a Parallel/Serial (P/S) converter 112. The aforementioned modules are disclosed only for illustrative purposes, and the OFDMA transmitter 100 may further include additional modules as necessary.
A signal processing in the OFDMA transmitter 100 will hereinafter be described in detail. Firstly, a bit stream is modulated into a data symbol sequence by the constellation mapping module 102. The bit stream is obtained from a variety of signal processing operations on a data block received from a Medium Access Control (MAC) layer. For example, a channel encoding, an interleaving, a scrambling, and the like may be applied on the data block received from the MAC layer. The data block may also be referred to as a transport block as necessary. A modulation scheme may be decided in consideration of a channel status, a buffer status, a required Quality of Service (QoS), and the like. However, the modulation scheme may further include but not limited thereto a Binary Phase Shift Keying (BPSK), a Quadrature Phase Shift Keying (QPSK), and n-Quadrature Amplitude Modulation (n-QAM). After that, a serial data symbol sequence may be converted into Nu parallel data symbol sequences by the S/P converter 104. Nu data symbols are mapped to Nu allocated subcarriers from among all the Nc subcarriers, and the Nc-Nu remaining subcarriers are each padded with ‘0’ by the symbol to subcarrier mapping module 106. Then, data symbols mapped to a frequency domain are converted into time domain sequences by the Nc-point IFFT module 108. After that, in order to reduce an Inter-Symbol Interference (ISI) and an Inter-Carrier Interference (ICI), the cyclic prefix module 110 generates OFDMA symbols by adding a Cyclic Prefix (CP) to the time domain sequences. By the P/S converter 112, the parallel OFDMA symbols are converted to a serial OFDMA symbol, and the serial OFDMA symbol is transmitted to the receiver after passing through necessary processes. Available subcarriers among the Nc-Nu remaining subcarriers, that have been left after being used by the former UE, are allocated to the latter UE, such that the latter UE transmits data through the available subcarriers.
As shown in FIG. 1B, a receiver 120 includes a serial/parallel (S/P) converter 122, a Nc-point Fast Fourier transformation (Nc-point FFT) module 124, a subcarrier to symbol mapping module 126, a Parallel/Serial (P/S) converter 128, and a constellation demapping module 130. The order of signal processing steps of the receiver 120 is opposite to that of the transmitter 100.